The present invention deals with the manufacturing process of casting various configurations of tubular hollow fiber membranes, for example, membranes that have a 6 millimeter (mm) diameter tubular shape with a wall thickness of approximately 0.125 mm. Such membranes may be used in an artificial pancreas, as described by Chick et al. in U.S. Pat. No. 5,002,661, issued Mar. 26, 1991. These membranes, however, are very fragile and difficult to form. It is particularly difficult to form tight coil and spiral shapes with such a wall-to-diameter ratio.
A known casting method comprises forcing a polymeric fluid out of an annular orifice of a nozzle of a coextrusion die while forcing a coagulating fluid out of a center core of the nozzle. A phase inversion process occurs at the point of contact between the two fluids; as a consequence, the polymeric fluid coagulates and forms a tubular structure. The tubular structure's coagulation rate is mitigated by solvents in the coagulation fluid, which allows for some manipulation of the fiber before final hardening occurs. A large bath of water is present a few millimeters below the extrusion die. The forming tubular structure is immersed into the bath, where further coagulation commences, and formed into the desired configuration before it can become fully congealed.
In certain applications, it is desired to use hollow fiber membranes in specific shapes or configurations, for example in a spiral or coiled shape. It is important, however, that no distortions or crimping occur in the membrane wall.